This invention relates generally to a sawing machine and more particularly to an improved press roll system for a sawing machine of the type used for longitudinal cutting of pieces such as cants, side boards, or slabs.
In a typical sawmill it is common practice to break a log down at the primary breakdown station into a plurality of elongated pieces. The center piece is typically called a center cant and has two parallel surfaces with the two edges being portions of the log circumference. Usually side boards are also produced at the primary breakdown station and likewise include a pair of parallel surfaces together with a pair of curvilinear edges. Slabs can also be produced at the primary breakdown station and are comprised of pieces having one flat surface together with a curvilinear surface comprising a portion of the log circumference. In further processing of the pieces coming from the primary breakdown station, it is common practice to position the workpiece with one flat side on a reference conveying surface for feeding through the secondary breakdown station which may be comprised of a typical edger, rotary gang, sash gang, or the like.
The function at the secondary breakdown station is to produce generally elongated boards having rectangular cross sections. A typical rotary gang sawing machine will have a plurality of saws mounted on an arbor for making multiple longitudinal rip cuts in a cant, thereby forming the side faces. A plurality of press rolls will normally exert a suitable amount of pressure on the top surface of the cant to hold it firmly against a bottom conveying surface. In a typical edger sawing machine, it is usually the purpose to square up the two opposing edges of an incoming piece to a selected width rather than to break it into a plurality of pieces as in a rotary gang. An edger type sawing machine usually has a pair of saws, one of which is fixed, with the other being laterally adjustable depending upon the maximum width that can be obtained from an incoming piece. Similar to a rotary gang sawing machine, an edger will have a plurality of top press rolls to constrain the incoming piece as it passes through the sawing station. A sawing machine that is suitable for producing a rectangular piece from a slab is generally similar to the just-described sawing machines, except that an additional saw or chipping head is provided in order to produce the additional horizontal surface or side face in order to form the resulting rectangular piece.
In the secondary breakdown sawing machines, when a decision has been made with regard to a cutting pattern for a particular piece, it will then be positioned with respect to a reference plane (usually a fixed saw and line bar) for feeding through the sawing machine. Once the piece has been positioned atop the infeed conveying surface, it is then apparent that accurate straight line travel through the sawing machine is a necessity. Since the sawing action exerts tremendous forces on the traveling piece, it is normally held in position by a plurality of top press rolls which exert a suitable amount of pressure on the top surface as it travels into, through, and out of the sawing machine.
In sawing machines of the type previously noted, the primary design criteria has been a high rate of productivity with a reasonable degree of sawing accuracy. In the past, it has been a normal occurrence that when the rate of production (feed rate) is increased, correspondingly a reduction in accuracy will result. By accuracy it is meant that after a piece is positioned with respect to a reference plane, it is then cut with the minimum degree of variation from straight and parallel lines of cut. With regard to increasing productivity while maintaining or actually increasing accuracy, little regard or attention has been given to the press roll system.
One cause of inaccurate cuts in existing systems is due to the fact that an incoming piece must physically lift those press rolls that then exert the constraining pressure on the top surface. At the high rates of speed of the incoming boards, as is required to maintain a high rate of productivity, the impact of a board hitting a press roll can cause a misalignment resulting in the problem with inaccurate sawing. This problem is particularly prevalent when small pieces are fed through an edger-type sawing machine where the impact of a small piece hitting a large press roll is very likely to cause a deviation from the intended travel path. When a plurality of press rolls is encountered during the feeding and sawing process, the misalignment may be compounded due to the board hitting several additional press rolls prior to the finished piece exiting the sawing machine. It is apparent that in such a situation the sawing accuracy is greatly affected, and a large amount of waste may occur due to improperly cut boards.
In the prior art, there are several examples of sawing machines that have a plurality of press rolls which are essentially automatically positioned in response to a sensed thickness of an incoming piece. For example, in the British patent 788,839, a press roll system is described wherein a thickness sensing arm detects the thickness of an incoming board which then serves to control the height of the press rolls above the horizontal feeding plane of the sawing machine. The position of the press rolls, however, is always such that their lowermost edges are slightly lower than the upper surface of the incoming board. With this system, the incoming board will still necessarily impact the press rolls in order to raise them, thereby affording an opportunity for misalignment of the board. Another example of a prior art system is that disclosed in the patent to McMillan, U.S. Pat. No. 3,742,992, in which the thickness of an incoming piece is sensed in order to set a plurality of press rolls.
One of the main problems with raising and lowering or sequencing of press rolls, while attempting to increase or maintain sawing accuracy, is to maintain a reasonable rate of production through the sawing machine. Accordingly, a sawing system that is to be designed for high accuracy sawing should also have as a design criteria the maintenance of a high production rate. It is therefore apparent that an improved press roll system should necessarily be designed for automatic operation. Suitable sensing means will detect the required parameters and then act to control the respective positions and sequencing for the press rolls. With such a system, one obvious design criteria is to provide the least amount of movement of the operating parts for a given incoming piece. In this respect, a memory means is desirable to recall the parameters of the preceding piece, such that if the next incoming piece has the same parameters, the press rolls will already be in position and thereby require little movement.
Another problem in prior art sawing systems having press rolls is that of misalignment caused by aberrations on an incoming piece impacting a particular press roll and causing misalignment. In several prior art systems, solutions to this problem have been suggested which generally include a flexible biasing means associated with the press rolls such that any shock load as a result of an aberration hitting the press roll will be effectively absorbed by upward movement of the press roll. It has been found in designing the improved press roll system for combined high-speed operation and elimination of misalignment caused by the board impacting the press rolls, that the aforementioned flexible bias means could be designed into the system by utilizing an air cushion system separating the individual press roll from its positioning mechanism. with such a feature, each press roll is allowed to rise individually in order to pass over knots or other irregularites that might occur on the top surface of an incoming piece. It is apparent that this feature will allow for the effective feeding and sawing of slabs or other miscut pieces of lumber which are being remanufactured.
Another feature that must be designed into sawing machines of the type utilizing press rolls is the continuous pressing function, primarily for safety purposes. It is essential that when a piece is being fed through the sawing station a minimum number of press rolls be exerting an appropriate amount of pressure to hold the workpiece in its path of travel. If an incoming piece is not properly held, there is a likelihood of saw damage and also creation of the extremely hazardous condition of lumber possibly being ejected from the machine. Thus it becomes a design criteria to provide that the minimum number of press rolls always be atop a piece being cut. This condition becomes somewhat difficult to maintain when incoming pieces of varying thicknesses are to be cut with no interruption in the feed rate. Again, it becomes necessary to provide a fast-acting, well-timed system to properly position and control in sequence the press rolls.
Due consideration must also be given to the complexity of the overall design in view of reliability and the available maintenance personnel in sawmills. Sawmills are notorius for their rugged operating environment, and any newly designed piece of equipment must necessarily be capable of reliable operation in the rough environment. Consequently, mechanical limit switches and hardwired control circuits are desirable. In addition to their reliability, such hardwired systems lend themselves to relatively easy maintenance in the sawmill environment.
Accordingly, from the foregoing, one object of the present invention is to provide an improved sawing machine press roll system for more accurate feeding of the pieces through the sawing station.
Another object of the invention is to provide a press roll system where each individual press roll will absorb the shock loading from contacting a protuberance on the piece.
A further object of the present invention is to design an improved press roll system that is essentially automatic in operation, allowing high feed rates through the sawing machine.
Still a further object is to provide a simple memory means whereby succeeding pieces of similar thickness can be accommodated with minimal movement of the press rolls, thereby reducing machine wear.
An additioonal object of the invention is to provide an improved press roll system and control therefor that will operate with a high degree of reliability in the rough environment of a sawmill.
Still a further object of the present invention is to provide an improved press roll system where maintenance can be performed by existing sawmill maintenance personnel in a minimum time period.
These and other objects of the invention will become apparent upon reading the specification to follow in conjunction with the attached drawing.